This invention relates to a video transmission apparatus connected to a network. More particularly, this invention relates to a video transmission apparatus for increasing a quantity of data that can be transmitted through a network.
A video transmission apparatus for connecting to a network converts video signals taken by an imaging device such as a camera from analog signals to digital signals (A/D conversion) and acquires video data. Each frame of the video signal constitutes this video data, and data compression is made by a video coding system such as JPEG (Joint Photographic Experts Group), for example. The compressed video data so obtained is transmitted to a reception apparatus connected to a transmission medium such as network typified by LAN (Local Area Network) and WAN (Wide Area Network). Further, the video transmission apparatus for use with a network receives the compressed video data from the transmission medium such as the network, expands the compressed video data and outputs video data or displays it on a monitor.
To accomplish a low cost and multiple functions, the video transmission apparatus for connecting to a network employs CPU (Central Processing Unit) for its data processing portion. For example, CPU reads the compressed video data from a JPEG compression circuit that compresses the video data by the video coding system JPEG, and stores it in a storage device such as a memory. Further, CPU reads out the compressed video data stored in the storage device, formats it into a predetermined format, again stores the compressed video data so formatted and then transmits it at predetermined timing through the transmission medium to a network control circuit for controlling data transmission and reception. The network control circuit converts the formatted compressed video data to data that is reconstituted into a format corresponding to a communication protocol of the network, and transmits it to the network.
As described above, the data processing of CPU includes data transmission for acquiring the compressed video data from the JPEG compression circuit and storing the data into the storage device, data transmission for acquiring the compressed video data, formatting it to the predetermined format and again writing the compressed image data so formatted into the storage device, and data transmission for transmitting the compressed video data formatted into a predetermined format to the network control circuit. To conduct these data transmission operations at a high speed, some video transmission apparatuses employ a plurality of dedicated bus systems. Such a dedicated bus system includes a dedicated bus that enables CPU to simultaneously and independently exchange data with the JPEG compression circuit and with the network control circuit besides a bus system through which CPU gains access to data for an execution command of CPU itself and to data to be processed.
Recent CPU has a plurality of DMA (Direct Memory Access) transmission functions for executing data access with a built-in buffer and with external equipment independently of command execution of CPU itself, and a plurality of interfaces for exchanging data by using the DMA transmission functions under the state where no processing load is applied to CPU itself.
CPU can connect the JPEG compression circuit or the network control circuit by using this DMA transmission function and the interface through the dedicated buses. When a predetermined quantity of data is stored in the built-in buffer by the DMA transmission function, CPU reads the data from the built-in buffer, executes a predetermined data processing, and stores the data so processed into the storage device or simultaneously exchanges the data processed in a predetermined way with the network control circuit by using other DMA transmission function.